Problem: On the first 4 tests of her chemistry class, Stephanie got an average score of 80. What does she need on the next test to have an overall average of 84?
Explanation: Let her score on the next test be $x$ The sum of all of her scores is then $4 \cdot 80 + x$ The same sum must also be equal to $5 \cdot 84$ Solve: $x = 5 \cdot 84 - 4 \cdot 80 = 100$.